1. Field of the Invention
The present invention relates to the determination of properties of objects. More specifically, the present invention relates to a method and apparatus for detecting properties of gases and plasmas, as well as surface and sub-surface properties of materials.
2. Related Art
In aerospace engineering and other disciplines, it is often important to remotely monitor and determine properties of gases and plasmas, as well as surface and sub-surface properties of materials. For example, it is often desirable to remotely identify species of gases and plasmas, as well as to measure selected state populations, velocities, rotational and vibrational temperatures, thermodynamic properties, and nonequilibrium conditions which may exist in such gases and plasmas. Further, the ability to remotely determine properties of gases and plasmas is especially useful in studying hypersonic flows immediately behind a shock, in a wake, or in a boundary layer region where complex velocity fields exist and where nonequilibrium states and chemical reactions may be occurring. The ability to detect surface and sub-surface properties of materials is especially useful in medical imaging and semiconductor processing applications.
Resonant, Enhanced, MultiPhoton Ionization (REMPI) is a known, ultra-high sensitivity probing technique for detecting low concentrations of molecular species in gases. In REMPI, a single, tunable laser is used to ionize a region within a gas, and properties of the ionized region are analyzed. REMPI is an effective spectroscopic tool because the multiphoton ionization cross-section is strongly enhanced by resonant intermediate states. This enhancement can be due to single photon or multiphoton resonances. As a laser is tuned through these resonances, the ionization yield reflects the spectrum of the resonances, thereby providing an indication of molecular species in a gas.
FIG. 1 is a diagram illustrating the REMPI technique. The (n+m) REMPI process can be characterized by the absorption of n photons to a resonant state followed by the absorption of m additional photons to ionization, Thus, a first type of REMPI excitation process, which is referred to as a (1+1) process, is characterized by a single photon resonance (i.e., a single photon exciting from the ground state of an atom or molecule to one or more intermediate states) followed by a single photon exciting the atom or molecule from that intermediate state to above an ionization threshold. A second type of REMPI excitation process, which is referred to as a (2+1) process, is characterized by a two photon resonance (i.e., two photons exciting from a ground state to one or more intermediate states) and a single photon exciting to above an ionization threshold. Additional REMPI processes, including 3+1, 4+1, etc. are possible. A third type of REMPI excitation process, which is referred to as a (2+2) process, is characterized by a two photon resonance followed by two photons exciting to above an ionization threshold. Additional REMPI processes with multiple photons required for ionization are also possible. In each of these cases, the photon resonance energy and the energy required for ionization can be analyzed to determine properties of the gas or plasma under study. The selection of which of these (n+m) REMPI excitation processes to use depends on the particular molecule and the available source laser. Usually, the intermediate state is in the ultraviolet portion of the electromagnetic spectrum, so the 1+1 process requires a tunable ultraviolet laser, but higher order processes such as the 2+1 and 2+2 processes may be achieved using a visible or near ultraviolet source.
In conventional REMPI applications, low pressure gases are used and ionization is measured using electrodes or wire probes. A direct current (DC) potential sweeps charges out of the ionized region, which generates a pulse of current through a detector system when the REMPI-generated ionization occurs. REMPI has also been applied at higher pressures using small probe detectors and in flames using the conductivity of the flame, which is in contact with two electrodes. However, conventional REMPI applications are limited because of the need to sweep the charges out of the ionized region to facilitate detection. Further, electrodes or wire probes must be in physical contact with the ionized region, thereby preventing remote measurement and detection of properties of gases and plasmas.
A number of techniques have been developed for the measurement of velocities of gas flows. Velocity is a fundamental transport parameter in a gas flow, and its measurement is of primary importance both for characterizing the flow and for validating predictive models of the flow. Often, it is the velocity in a specific location that is of most importance, such as the velocity close to a surface, behind a shock, or in the wake of an airfoil. Laser Doppler Velocimetry (LDV) and Particle Imaging Velocimetry (PIV) are known techniques for measuring flow velocities. However, both limited by a random arrival of particles at a location of interest. In addition, these methods suffer from particle “slip” in high speed flows and near surfaces, where the particle density may be particularly low.
Flow tagging approaches have also been developed, based on vibrational excitation of oxygen, creation of NO, and other approaches. However, these approaches are not effective in high temperature environments where vibrationally-excited molecules are already present and/or radical chemical species may already be present. Velocity measurement by laser breakdown has been used, but it introduces large perturbations into the flow, and tracking of the breakdown by shadowgraph or schlieren limits this approach to flows with low complexity and simple geometries.
Temperature measurements in a high-speed flow and in combusting environments are always difficult to perform. For instance, intrusive probes perturb the flow or the combustion process. As a result, various non-perturbative approaches have been developed, including Laser Induced Fluorescence (LIF), Rayleigh scattering, and Coherent Anti Stokes Raman Scattering (CARS). However, each of these approaches has limitations. The LIF methods are subject to quenching errors and cannot easily be applied in air flows since there is no convenient fluorescing species. Rayleigh scattering is subject to interference from background light and relies on knowledge of the species mole fractions. In its most common implementation, a Rayleigh measurement is of a density and so pressure must be known and the ideal gas law used to convert to temperature. CARS measurements are quite complex, and proper fitting of spectral information becomes very difficult for complex gas mixtures.
For measurements of free carrier lifetimes in semiconductors without using contacts, present methods require that the semiconductor be illuminated with a pulsed or amplitude modulated laser to form the free carriers and the transient absorption be measured with a second light source. This method is limited to thin materials so the second light source is not substantially absorbed before passing through the material. It is also limited in signal-to-noise ratios by the requirement that the percentage of light absorbed be significant enough to be detected in the presence of the background shot noise from the illuminating light source. Additionally, for absorption measurements, optical access must be provided on both sides of the semiconductor, so the semiconductor cannot be mounted. The low signal-to-noise ratio requires that the detection process be integrated for long time intervals.
Accordingly, what would be desirable, but has not yet been provided, is a method and apparatus for detecting properties of gases and plasmas, and surface and sub-surface properties of materials, which addresses the foregoing limitations of existing monitoring techniques.